Radio systems are very important to our society. Police and law enforcement officers use mobile and portable two-way radios every day to stay in touch with dispatchers and with one another. Fire fighters find out about fires and receive calls for help over land mobile radios. Paramedics and ambulance drivers communicate with doctors and hospitals via radio communications. Our security and safety depends on these radio communications systems being reliable and cost-effective.
Some of the most challenging problems facing radio designers today relate to issues of "compatibility." Generally speaking, a radio is "compatible" with a radio system if the radio can successfully work with the system. Obviously, it would be a bad idea to give a police officer a radio that is not compatible with the police department's radio system, since this would cut off communications between the police officer and the rest of the police department.
Different countries of the world have developed different standards specifying how radios should communicate with one another. For example, the European Telecommunications Standardisation Institute (ETSI) has developed an open standard called "TETRA" that defines a radio air interface for European mobile radio users. See ETS 300 392-2 ed.1 (1996-03) entitled "Terrestrial Trunked Radio (TETRA): Voice plus Data (V+D); Part 2;Air Interface (AI). The TETRA air interface provides a four-slot, high speed digital data rate (36 kbps) time division protocol based on a certain European radio channel bandwidth (25 kHz). In contrast, the North American land mobile radio market is moving toward narrow band signaling. Many North American radio channels are only half as wide as the European channels (i.e., 12.5 kHz as compared to 25 kHz). Radios transmitting at the higher, European data rate will not work with systems designed for the narrowband North American standards because they will exceed the maximum bandwidth for a narrowband channel.
Additionally, ETSI's TETRA standard specifies the use of a speech vocoder compliant with certain European vocoding standards. Basically, a vocoder ("voice coder") is a device that converts speech signals into compact digital signaling for transmission. Vocoder standards developed by the French company Thomson-CSF Communications Corp. have achieved wide acceptance in the European radio market. In contrast, the North American land mobile radio market has standardized on a different vocoder standard called "Improved Multi-Band Excitation" ("IMBE"). See APCO Project 25 Vocoder Description (Standard No. IS-102.BABA, Telecommunications Industry Association). The Thomson vocoder standards are based on a certain vocoder frame duration (i.e., 30 ms), whereas the North American IMBE standard is based on a different vocoder speech frame duration (20 ms).
For many years, mobile radio users in North America and elsewhere have achieved great success with Ericsson's Enhanced Digital Access Communication System ("EDACS"). The EDACS system provides frequency division type signaling on control and working channels. Existing land mobile radio customers such as those using Ericsson's EDACS system may wish to migrate to the benefits of time division signaling--but may want to use signaling that is similar to the European TETRA standard while using a different vocoder technology, such as IMBE and/or a different data transmission rate suitable for use with North American narrow band channels.